White Roofs May Actually Add to Global Warming

By Douglas Main | October 20, 2011 8:53 am


A new study published in the Journal of Climate claims that painting rooftops white—a method championed by energy secretary Steven Chu and others to combat climate change—only minimally reduces local cooling, and actually causes a slight increase in overall global warming.

How the Heck:

  • The researchers used a global climate model called GATOR-GCMOM [PDF], which incorporates a host of data from satellites and weather stations worldwide. It models how relationships between various environmental conditions, like the presence of clouds or pollutants, will affect local and global climate.
  • The model found that more white roofs means less surface heat in cities (which is obvious enough to anyone who’s sat in a car with a black interior in the sun). Lower local temperature means less water evaporates and rises up to eventually form clouds, says lead author and Stanford University researcher Mark Jacobson. The decrease in clouds allows more sunlight to reach the Earth’s surface, leading to higher temperatures overall.
  • The model also predicts that much of the light reflected by rooftops will eventually be absorbed by dark carbon soot and particulates that are especially prevalent in the air above urban areas. This could limit local cooling and cause warming elsewhere as the particles drift away.

Not so Fast:

  • One possible benefit to white roofs is the reduction of cooling costs for the buildings painted white, which isn’t explicitly addressed in this paper. This could make the practice useful in warmer climates, but at least one study has found that a switch to white roofs wouldn’t lead to energy savings on a global scale.
  • The study didn’t calculate how the change would impact energy use, or how such a change could impact emissions and their effect on climate.
  • Even with switch to 100% white roofs, the predicted increase in global temperature (0.13 F over 20 years) is quite small, and dwarfed by expected effects of greenhouse gases and carbon soot.
  • Data generated by this model are preliminary, as with any computer simulation of a system as vastly complicated as Earth’s climate, and the matter is far from settled. Other researchers stand by their own calculations that white roofs can provide energy savings in a variety of climates and reduce heating of the Earth’s surface.

To Paint or Not to Paint:

  • Jacobson speculates that any energy savings in white-roofed buildings would be eaten up by increased energy use elsewhere (i.e., for cooling) from overall warming caused by white roofs. So it’s probably not a great measure for widespread use, he says. But in a warm, sunny climate, a white roof almost certainly doesn’t hurt on an individual basis and may help reduce the need for air-conditioning (as inhabitants of sultry climes have known for a long time).
  • Conversion to white roofs is a bad idea globally, Jacobson says. Instead, if you want to make a difference, install a photovoltaic system or solar panels on your roof, which reflects light and also generates clean electricity.

Reference: Mark Z. Jacobson and John E. Ten Hoeve. Effects of Urban Surfaces and White Roofs on Global and Regional Climate. Journal of Climate. 2011. DOI: 10.1175/JCLI-D-11-00032.1

Image: Christopher Dick / Flickr

CATEGORIZED UNDER: Environment
  • Read H.

    Coffee is good..coffee is bad.
    Cranberries cause cancer. Cranberries are safe.
    Acid rain – press gave up
    Global cooling – short term trend, press gave up
    Ozone hole – press gave up
    “global warming” morphs into “climate change” morphs into “global climate disruption”. What is next?

    We can’t even predict cloud cover to within one square mile, one hour into the future with any accuracy. When will the Left abandon ship here and move on to the next scare?

  • Tom t

    I thought the science was settled, but they don’t even understand as simple a thing as this. Why on earth would we spend trillions to stop global warming when we aren’t even sure what will or will not work.

  • JeT

    @Tom t,

    Agreed. Also, I don’t know how I’m going to die, so I’m not going to make any effort to eat healthy food or exercise. I also don’t plan on learning anything new either, since that will just be more wasted effort, since anything I learn about or experience might lead to my eventual death.

    Seriously, though. The science of global climate is pretty tricky. Some things scientists are very sure about, and others they’re still working on. We don’t have all the answers, but we do have some of them, and not everything involves spending trillions of dollars.

  • AJC

    Well, it seems like everything causes warming, no matter what it is. Its a wonder the oceans aren’t boiling over. Also, its a wonder the poles are as cold as they are, what with all the white snow and ice reflecting sunlight causing warming. In addition, I remember not too long ago another great Journal article coming out saying if the Arctic melts any more the reduced amount of white ice will decreased the light reflected back to space and cause significant warming causing the Arctic to melt more. Well one of these groups is wrong. This just goes to show what happens when you rely on models to make decisions. And by the way, solar panels don’t reflect sunlight. They are designed to be good absorbers to maximize their efficiency. If they reflected sunlight, what would they convert to electricity.

  • John Moore

    @ Tom – While I agree with you I think that what is refered to as “the science is settled” has more to do with the fact that the planet is currently in a warming trend. Why is where we get into differing causes. Then the worst of all is how to help/correct/minimize to situation. No one knows that for sure….

  • Jeff L

    The problem with “Lower local temperature means less water evaporates” is that in most cities, rain water just runs off the roof and into the storm sewers. Once there, it doesn’t evaporate either. Also, most black roofs are made of asphaltic or other petroleum based materials (tar, EPDM, bitumen, etc). Water runoff from these roofs will contain various hydrocarbons, which will end up in the storm drains.

  • Chris

    What if I put aluminum foil on my roof, that would reflect all the light away, probably more efficient than white paint.

  • Geoffrey.Frasz

    I cannot help but to think of the brilliant white painted buildings in the Aegean Islands. Perhaps indigenous people have discovered solutions to heating and cooling that are appropriate for their geographical areas long before “science” tells them what to do.
    And BTW: Chris, if you put aluminum foil on your roof you will also block the mind control rays the government (or Rupert Murdoch) is beaming at you. :)

  • podunkmo

    That aluminum foil is a good idea. Also protects you from the alien radio signals beeming in from space.

  • http://www.rationalpastime.com/ J-Doug

    This: The problem with “Lower local temperature means less water evaporates” is that in most cities, rain water just runs off the roof and into the storm sewers.

    Also, I’m skeptical that the evaporative effect (or lack thereof) would have nearly as much of an impact as the reduction in energy consumption from running air conditioners.

  • Jeff Patterson

    The pin heads have been telling us that the reason GHGs which make up .03% of the atmosphere can nonetheless have a significant effect on climate is due to positive feedback. But if less surface heat means less evaporation which means less cooling, then GHG warming means more evaporation = more cooling, i.e. negative feedback. The principle I guess is this: Any change in the environment which doesn’t require the dismantling the free market is deemed insignificant due to negative feedback effects while those that may help to further the greeny’s socialist agenda seem to go undergo catastrophic positive feedback which if left unchecked will end life as we know it

  • http://kforcounter.blogspot.com Cody

    It’s odd that people bring politics into this, attack free markets and put science in quotes.

    After reading the headline I expected the problem would be the manufacturing of the paint—the actual cause is much more interesting (subtle & sophisticated).

    It is a very difficult problem and we are working very hard to figure it out, those who simply fight it because of some misplaced idea that it’ll hurt our economy (which they also don’t understand) are invited to propose alternative methods of problem solving, because from where I sit, science is our only hope.

    In the mean time please return all of the fruits of science you enjoy (virtually all technology, electronics, medical knowledge/treatment/equipment, advanced textiles, cars, computers, many synthetic materials, industrial farming, etc.).

  • Luke

    “Conversion to white roofs is a bad idea globally, Jacobson says. Instead, if you want to make a difference, install a photovoltaic system or solar panels on your roof, which REFLECTS LIGHT and also generates clean electricity.”
    ———————————————
    Uh, what the heck do you think a white roof does? Reflects the visible spectrum, along with some of the low-frequency infrared. Real world testing of Roof Arrays with varying grades of tint, have proven what the ME has known for centuries. White roofs are MUCH cooler. To suggest that it is a mistake to use them because it might raise the temperature in the Himalayas, (unproven) is absurd. Probably the dumbest story I’ve ever seen in Discover. See: Florida Solar Energy Center

  • Monkey

    beeming?

    beaming.

  • Monkey

    look, there are two arguments here.

    Just because we dont know HOW to fix a problem doesnt mean that the problem doesnt exist. If I have no clue what will cure my cancer, it does not logically follow that I therefore dont have cancer.

    Climate is changing. Fact.

    What to do. Multifaceted discussion … subject to change.

  • Stephen Teal

    While a white roof in the southern states will reduce cooling loads and therefore save energy even some white roof manufacturers agree that the benefits of white roofs in the northern states an especialy in most of Canada are negligible at best and even an energy negative in many areas. In the north the only minor benefit may be a slight reductioin in heat island effect, but that only applies while the roof is clean and in urban areas. Has anyone thought about what happens to all the cleaning solutions that will be used to keep the roofs clean? I know in many cities they will end up in the river/lake via the storm sewers, untreated.
    While programs such as LEED promote white roofs as a blanket policy people need to think the process through, if you have 10 heating degree days for every 1 cooling degree day what is more inportant. In the north most buildings have much higher insulation values negating many of the possibile benefits of the reflective roofing. With a 50 storey building with precast concrete walls, black parking lot etc how significant is the roof area?
    When the heat is reflected off the roof where does it reflect to, through a window?
    Come on people think the process through this is not rocket science.

  • Tim NM

    At the end of the day, having a white roof cannot hurt. It does keep buildings cooler which translates into lower energy costs. That translates into the opportunity to use that energy on other efforts.. We are going to use the energy, the question is when, and on what.

    Low hanging fruit such as white roofs, more energy efficient buildings, and a better understanding of building envelope science can lower energy usage and costs. Storm water run-off is another issue but is related. Hey if they can clean the poop out of drinking water, taking out the hydrocarbons and aromatics of the roof run-off should not be a challenge when the need for clean water dictates that it be done

  • SchreckenLicht

    One thing that does work to effect some local cooling is to have plenty of trees around buildings if possible. I have a grey shingle roof but the sun rarely hits it in the summer due to all the deciduous trees on my property. This isn’t practical in all circumstances but it is easy to do and doesn’t require onerous taxes and fees levied by governments in the name of attempting to alter the climate. Heat islands in urban areas are in part caused by lots of blacktop and little shade and too few trees, among other reasons.

    In addition, white roofs aren’t always practical, especially because many people have ordinary shingle roofs and while shingles come in very light colors, I’ve never seen any that are white. That, and as has been mentioned, keeping them clean!

  • Roofer EP

    One thing is key in this – white roofs may reduce cooling costs. What about 75 percent of the US where there are more heating degree days (HDD) than cooling degree days (CDD) and a dark colored roof actually provides more benefits by reducing the heating costs…

  • SMenon

    The recent paper by Stanford’s Mark Jacobson and John Ten Hoeve (2011) on urban heat islands and cool roofs is a useful contribution to the literature. However, their results regarding white roofs are preliminary and uncertain. Along with our own work at the Lawrence Berkeley National Lab, other published papers have addressed the broader benefits of white roofs. In our view, these studies taken together raise important issues that need to be considered from a policy standpoint to fully understand the potential of more reflective (white or cool) surfaces.

    Jacobson and Ten Hoeve note that reflecting light from white roofs may lead to a decrease in cloud cover, thereby increasing, not decreasing, the urban heat effect. But they also note that their findings might change if they used different models. This is an ongoing research area not only for their group, but others, and ours as well. The findings should not be considered settled.

    We have found that white roofs do provide a low-cost solution that can help buildings reduce energy costs, in a wide variety of climates, as well as cool the atmosphere regionally and globally. We have also found disadvantages. The reflective roofs may cause unwanted glare, for example, and may modestly increase heating costs in winter. But answers to these issues are exactly the ones we’re working hard to find.

    Our work has shown that reflective roofs can lead to better air quality, reduce the strain on our electrical grid, improve comfort and decrease emissions from power plants. These are important considerations when evaluating all the available research.

    In our opinion, all of these arguments and studies suggest that selective use of white and other reflective roofs makes sense as part of an integrated strategy for more sustainable human existence on Earth. But the potential benefits offered by cool roofs do not diminish the need for sustained reductions in greenhouse gas emissions to control global climate, or the need for increased use of renewable energy sources.

    It’s important for the public to understand that scientific debate leads to better science. But it’s also important that the public receives—and the news media delivers—a message that properly conveys research news with all its many caveats and cautions. It’s not settled, until it’s settled.

    For more information, go to: http://heatisland.lbl.gov/news/cool-roofs-and-global-cooling-heat-island-group-responds-jacobson-ten-hoeve-2011

    Surabi Menon and Ronnen Levinson
    Berkeley Lab Heat Island Group

  • Douglas Main

    Surabi: Thanks for your comment. The uncertainty is important to emphasize. I’ve added the following two sentences (under “Not So Fast”) and linked to your group’s response to the study: “Data generated by this model are preliminary, as with any computer simulation of a system as vastly complicated as Earth’s climate, and the matter is far from settled. Other researchers stand by their own calculations that white roofs can provide energy savings in a variety of climates and reduce heating of the Earth’s surface.”

  • David

    16. Stephen Teal

    …When the heat is reflected off the roof where does it reflect to, through a window?
    Come on people think the process through this is not rocket science.

    Hi Steve interesting point
    In an urban environment you also have thermal mass to take into account, I don`t have the mathematics to put here, however a simple analogy would be if you painted a piece of concrete black and put it in the sun and had another identical piece painted white, the black piece would still be radiating heat longer after the sun has gone down.
    Dry air radiates heat into space very quickly (far less so with damp air) that is why deserts have such a swing in temperature from day to night.
    There are of corse a lot of other particles (man made usually as noted in the article) in air that can prevent that happening, that are found in urban in environments that are missing in deserts, so this is very patchy science I am presenting.
    But at least on the heat loss of dry air compared to heat loss of moist air, there is some good science to back it up.
    Most urban environment are made of high thermal mass products, Concrete, Bitumen Steel, these all will heat up more if they are darker colors and radiate back into the local environment after dark.
    Where as if they radiated heat back into the air instead of absorbing it all day, there might be a better chance of a cool down before the sun comes up the next day.
    As the article makes clear it is a lot more complicated than that, but it’s learning process

  • http://anthonymaturin.netnz Anthony Maturin

    White roofs? Muy interesante. BUT, don’t allow arguments like these deflect us from the need to take responsibility for our personal emissions in all areas, and to reduce them to our global share. Something less than two tonnes p.a. I believe just now. My own observation is that most of the responsible thinking comes from outside the US. The worst case scenarios of four degrees of warming by mid century are not pie in the sky, and although not yet thought of as probabilities by many people, they are still distinct possibilities and certainly to be taken into account when formulating personal approaches and national policies.

    A world with vastly shrunken inhabitable land mass, with its forced movements of millions of CC refugees, including I might add internal refugees in the US, is a disturbing picture. One that we MIGHT avoid if we pull our fingers out, beginning now. My concern is that concepts like the controversial white roofs one can too easily deflect some from the necessity of attaining carbon neutral societies by mid century at the latest.

    People around the Globe have made a start – China and the US included. But we can’t afford to be diverted by controversial techno fixes.

  • Greg

    Water does not have to be hot to evaporate. The majority of cloud formation is from the evaporation of water in the oceans and natural bodies of water, not from drain water mixed with bird fecal matter, and petroleum oil. The contaminents in this drain water make it less likely to evaporate and become clouds. This is a terrible article which was probably written by a proponent of the roof manufactureing companies to make up a black eye on the cool roofing industry. Roofers, and roofing manufactures HATE the cool roofing movement, because they dont want roofs to last, Why take something that lasts usually 12-15 years and push it past the 20 year mark? Obviously to save your money, but they want tear offs, they need that tear off money. I could be wrong, but that info combined with what seems their limited knowledge of anything dealing with science, especially PV solar panels, leads me to believe they are on the money’s side. There is a difference between solar reflectance and thermal emittance, but whatever you do dont actually research the topics before you divulge mediocre information based on heresay and undergraduate work that is non-published. I understand that no one knows for sure, but if you familiarize yourself with scientific facts, things become clearer, dont listen to me, its not like i have a bachelor’s degree in biology(minor in chemistry), nor have I been a certified Florida roofing contractor, specializing in cool roofing for the last five years. Nor has the company i own done more than 7 million square feet of cool roofing… Oh wait, that is me.

    I like how they blast Steven Chu, a nobel prize winner. Why not take a bite out of this quote: from the 1997 nobel prize winner Jody Williams, “If maintenance is deferred, buildings and structures decay to the point that major capital outlay is required.” You paint exterior walls so you wont have to replace them, why not your roofs? I guess lets leave the oil dependancy alone, and we will just keep using the same methods we have always used, which is what they want us to absorb from this “article.” Even though everything else in the last 200 years has advanced technologically, besides roofing, does not mean that it shouldn’t. If we do not advance as a civilization in all manors, then we are not truely completely advancing. Do not listen to these fools, read up on it.

  • Fred Twilley

    I question the claim that “light reflected by rooftops will eventually be absorbed by dark carbon soot and particulates that are especially prevalent in the air above urban areas” That light has already passed through the same air on the way down with out much loss in intensity. Reflected light is full spectrum light and is much less absorbable than the infrared light that is emitted much more from dark roofs. Most of the heating of surface air is from infrared light emitted from surfaces that converted it from the full spectrum light from the sun.
    Their must be a flaw in the computer model used. What is needed are some carefully designed experiments to sort out good science from junk science.

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